As distributed generation systems become part of the power grid, islanding is becoming an increased safety hazard for personnel and damage risk for grid-connected equipment. Islanding refers to a condition in which a distributed generator (DG) continues to power a segment of a distribution network or grid even though electrical grid power from the electric utility is no longer present. As shown in FIGS. 1A and 1B, the islanding phenomenon happens when the grid is intentionally or accidentally disconnected from the network and the DG continues to energize local loads. FIG. 1A shows a grid network 100 before islanding occurs and FIG. 1B shows a grid network 100 after islanding occurs. As seen from FIG. 1B, when an islanding condition exists, the main power system 102 is disconnected from the rest of the grid network 100 by the disconnection 104. The main power system 102, for example, may be a power source provided by a utility company, an electricity cooperative, permanent or semi-permanent generation, etc. When an islanding condition exists, as in FIG. 1B, the DG units 106 will feed the load 108 unless the DG units cease to generate power.
Aside from the danger to maintenance personnel arriving to service a circuit that is energized by DG systems, also called a feeder, there are also operational issues due to islanding. IEEE 1547-2003 standard stipulates a maximum delay of 2 seconds for detection on an unintentional islanding condition and all DG systems 106 are required to cease energizing the load network, which may be a power grid. The islanded system may also be insufficiently grounded by the interconnection inside the DG. Re-closure operations that are initiated by the utility to clear the fault or disconnection 104 may also cause large mechanical torques, along with currents, particularly at in-rush, which are harmful for equipment in the islanded network.
A common example of islanding may occur at a grid supply line that has one or more solar photovoltaic (PV) power plants or systems attached to it. In the case of a blackout, the solar PV systems will continue to deliver power as long as there is sufficient sunlight. In this case, the supply line becomes an “island” with power surrounded by a “sea” of unpowered lines. For this reason, inverters for solar PV systems and other distributed generation (DG) systems generally have some sort of automatic anti-islanding circuitry in them.
Islanding detection methods can be classified into two major groups: remote and local methods. Remote techniques are based on the communication between utilities and DG systems such as power line communication, and supervisory control and data acquisition that do not have non-detection zones (NDZ), but are expensive to be implemented and therefore uneconomical. NDZs are defined as a loading condition for which an islanding detection method is unable to detect islanding. Local techniques, which are related to the DG, can be classified into two major categories: passive and active methods. Passive methods are based on measuring local parameters of DG and comparing the parameters to a reference value. Some commonly applied passive methods are over/under frequency protection (OFP/UFP), over/under voltage protection (OVP/UVP), phase jump detection, voltage harmonic monitoring and change in grid impedance detection. While these methods are simple to implement, typically, they fail to detect islanding in one or more powering/loading condition(s) leading to NDZ(s) for these methods. NDZs exist for OVP/UVP or OFP/UFP methods when the inverter generated power closely matches that of the load and, for the phase jump detection method when the load power factor is unity.
Active methods strive to reduce the NDZs associated with typical passive methods by adding field quantities, such as voltage, current, perturbations to the inverter. Some active methods include: (i) Output power variation method requires multiple DG systems but it fails when synchronization is not met due to the averaging effect; (ii) Active frequency drift (AFD) method requires adding small increments/decrements in the frequency of the inverter output current while monitoring the frequency of the voltage. AFD fails to detect an islanding condition when the load phase angle matches the phase offset of the perturbation. Sandia frequency shift (SFS) method which is an active frequency adjustment improves the performance of the AFD method by adding positive feedback to adjust the frequency away from the nominal value faster than the AFD method. However, there remains a need for systems and methods that are cost effective and efficient at detecting whether an Islanding condition exists.